The present invention relates to a vehicle control apparatus, and more particularly to a vehicle control apparatus for controlling a prime mover such as an engine and wheel brakes which are equipped in a vehicle with an automatic transmission.
A braking force retaining apparatus is known, which is equipped with a solenoid valve in a brake fluid pressure passage. The brake fluid passage extends between a master cylinder and a wheel cylinder. The solenoid valve can shut off the brake fluid pressure passage to retain the brake fluid pressure within the wheel cylinder after the depression of the brake pedal is released. This kind of braking force retaining apparatus provides a hill holder function to facilitate a starting operation on an uphill slope because the brake fluid pressure is retained after the release of the brake pedal. To realize the hill holder function, the brake fluid pressure is retained or released in accordance with various conditions.
U.S. Pat. No. 6,439,675 discloses a braking force retaining apparatus for decreasing the braking force that is retained without consideration of the brake pedal operation amount, wherein the retained braking force is decreased by an operation of the accelerator pedal, an engagement state of the clutch, etc. As one of the conditions where the braking force retaining apparatus decreases the braking force, it is recited that the transmission is set in the neutral (N) range.
The braking force retaining apparatus does not usually perform to retain brake fluid pressure in the neutral range. This is because the neutral range is a range where the driver has no intention of driving the vehicle with the engine driving force and where the vehicle is allowed to move by an external force or inertia, for example, in the case of traction. Therefore, it is considered that there is no need to retain brake fluid pressure when the driver selects the neutral range.
Meanwhile, a prime mover stopping apparatus is proposed, which stops the prime mover while the vehicle stops, to improve the fuel economy.
The prime mover stopping apparatus automatically stops the idling of the engine while the vehicle stops and restarts the engine when the driver depresses the clutch pedal or when the driver releases the brake pedal at a start up of the vehicle.
In cases where a vehicle with a fluid-type automatic transmission includes a braking force retaining apparatus and a prime mover stopping apparatus, the prime mover stopping apparatus automatically stops the engine when the vehicle that has been running stops in accordance with the depression of the brake pedal. Thereafter, as shown in FIG. 1, when the driver switches the selector of the automatic transmission to the neutral range and then releases the brake pedal, the retained braking force is released all at once. The prime mover stopping apparatus restarts the engine at the same time as the driver releases the depression of the brake pedal.
The fluid-type automatic transmission does not transmit the engine drive power to the driving wheels in the neutral range. However, a remaining clutch torque may exist in accordance with the oil pressure releasing time characteristics of the fluid clutch and/or the viscosity of the automatic fluid. Particularly, at a low temperature at which the automatic fluid provides a high viscosity, the transmissibility tends to be higher even if the transmission is set in the neutral range.
In this instance, if the engine restarts while the vehicle stops on a flat road with the automatic transmission set in the neutral range, a rotation torque of the axle may arise due to the engine torque that occurs at the restart of the engine and the remaining clutch torque of the automatic transmission, thereby resulting in dragging of the vehicle.
It is an object of the present invention to overcome the foregoing drawback.